1. Field of the Invention
The present invention relates to a method for measuring an aberration of a projection optical system of a projection exposing apparatus for resolving a circuit pattern of a semiconductor device or the like, and in particular to a method making it possible to measure an aberration of a projection optical system even if light from an subject pattern does not form an image by a standard exposure because the pattern is too fine.
2. Description of the Related Art
FIG. 1 is a schematic view illustrating a conventional projection exposing apparatus. An excimer laser beam emitted from an excimer laser source 1 is reflected on a mirror 2 to advance into a fly eye lens 4 through a shutter 3 for controlling an exposure. The beam whose illumination intensity has been made uniform through this fly eye lens 4 is restricted with a variable opening iris 5, and then passes through a lens system 6. The beam is reflected on a mirror 7 and is applied through a condenser lens 8 to a mask 9. The beam which has passed through the mask 9 focuses, through a projection lens 10, onto a wafer 11 on a stage 12.
As shown in FIG. 2, in this case a resist film on the wafer 11 is exposed to the beam, using a mask wherein a line-and-space pattern (referred to as an LS pattern hereinafter) is formed. The LS pattern is composed of plural lines (for example, L1-L5) which have the same width and are arranged in parallel and at even intervals. The exposed resist film is developed. About the LS pattern resulting from the development of this resist film, the widths of the respective lines thereof are measured. An aberration (i.e., aberration amount) of a coma can be obtained from the difference between the widths of the lines at both ends of this LS pattern (Japanese Patent Application Laid-Open No. 10-232185).
Conventionally, the difference between the widths of the both-end lines can be measured in an LS pattern having a line width of, for example, 0.25 .mu.m by exposure using, as a light source, a KrF excimer laser having a wavelength of 248 nm. However, the difference between the widths of the both-end lines cannot be measured in an LS pattern having a smaller line width, for example, a line width of 0.15 .mu.um.
In recent years, it has been demanded that a finer pattern be formed for a wavelength used in a projection exposing apparatus. For example, it is demanded that a pattern having a width of about 0.15 .mu.m be formed by exposure to a KrF excimer laser beam having a wavelength of 248 nm. In order to enlarge focal depth, therefore, it is necessary to use a super-resolution method such as annular illumination. However, if the difference between the widths of the both-end lines in an LS pattern having a width of 0.15 .mu.m is actually attempted to be obtained by the annular illumination, the both-end lines L1 and L5 may become highly thin to fall down by light approaching effect according to exposure using an optimal exposure, wherein the centerline L3 in the LS pattern is resolved to a line width as designed.
According to normal illumination under the illumination condition that the use of a phase shift mask is optimal: .sigma.=0.3, an LS pattern having a width of 0.15 .mu.m is not resolved by an optimal exposure of the LS pattern. In other words, if the centerline of the LS pattern is exposed to light using the optimal exposure, the both-end lines in the LS pattern are not resolved.
Hitherto, therefore, it has been unable to obtain a coma by the difference between widths of the both-end lines in the case of a pattern having a fine pattern line width such as 0.15 .mu.m.
Incidentally, the difference between widths of right and left lines changes correspondingly to exposures. Thus, it is necessary that the exposure as a standard be beforehand decided. Since the optimal exposure for the centerline of any LS pattern is not easily affected by aberration, the optical exposure for the centerline of any LS pattern is usually used as the exposure that is the standard.